Immersed membranes are used for separating a permeate lean in solids from tank water rich in solids. Typically, filtered permeate passes through the walls of the membranes under the influence of a transmembrane pressure differential between a retentate side of the membranes and a permeate side of the membranes. Solids in the tank water are rejected by the membranes and remain on the retentate side of the membranes. Despite the apparent simplicity of this process, however, two related issues have proven to be consistently difficult to address. The first issue relates to controlling the concentration of solids in the tank water. The second issue relates to the need to clean the membranes to prevent their rapid and sometimes irreversible loss of permeability.